They look for all the world like nice people. What with Brad Pitt in his smart-casual lounge suit and Angelina Jolie in her trad dress and brushed-back hairdo. But oh no, really they are cold-hearted killers, smirkingly focused on their cruel objective, each with a steel-tipped bullet - heading straight for Jennifer Aniston's heart. Ha! Ha! Ha!

Poor old Jen. As Brad's Pitt's wife, she got to be the wounded party in the sensational tabloid fallout from this occasionally entertaining, if very overlong, action comedy about two professional assassins living a chintzy married life in the 'burbs, whose real lives are a secret from each other until they are commissioned to smoke their respective spouses. And as all the world knows, Brad and Ange did the nasty during the shoot and perpetuated the association afterwards, thus breaking the lenient Hollywood rule of DCOL: Doesn't Count On Location - the movie actors' equivalent of the holiday romance.

It's a rule whose subsidiary clauses state that famous attractive people have something in common with the rest of us: they are attracted to famous attractive people. So when you see two pretty stars playing opposite each other on screen, especially somewhere necessitating a six-week hotel stay in, for example, New Zealand or Tunisia - well, without signed affidavits to the contrary, you have to assume they have also got temporarily involved. The only really enjoyable part of Oliver Stone's Alexander was a fabulously suggestive role for Angelina Jolie playing Colin Farrell's mum somewhere in the Mediterranean. That was a corking DCOL rumour in the making.

This movie, though taking up much less cultural space than the gossip, is not all that terrible: like a low-rent Prizzi's Honour, or high-rent True Lies, or a very diluted and sentimentalised version of The Wars of the Roses, Danny DeVito's acrid account of a sociopathically bad marriage.

These two A-listers model that sleek cat-that-got-the-cream look, droplets hanging off the whiskers, that Steven Soderbergh made compulsory for caper-comedy. Brad Pitt is developing a curious sort of middle-aged spread that appears to be affecting his face; his cheekbones are getting further and further apart, as if something very heavy is pressing down on his head. Angelina Jolie looks at all times fascinatingly and authentically barking mad; her lips still have that extra-terrestrial extravagance, but she is actually reasonably human in this film, giving subliminal but perhaps illusory hints of a sense of humour.

Mr and Mrs Smith begins with the pair having a marriage guidance session - with the counsellor out of shot - uncertainly insisting that their relationship is fine, and they just need a "tune-up". Later, each appears individually to confide their doubts. "It seems like there's a huge space filled with everything that we don't say to each other ... " says Brad, " ... what do you call that?" "Marriage," says the therapist.

A way of looking at this film, a generous way perhaps, is that it's a satirical metaphor; marriage is a secret, intimate combat in which the point is precisely to kill each other. The couple actually get to dance a symbolic tango in an exotic Manhattan restaurant, before ultimately breaking into cartoon violence. Angelina confesses everything to her husband as they cynically swoop and dip. "Satisfied?" she finally asks archly. "Not for years," he snaps back. In fact, the double-killer image could even be a reactionary nightmare riff on the theme of the 21st-century dual-income-no-kids couple: the two freelance professionals plying their lucrative trade in return for a sterile and essentially mendacious existence and inevitably turning on each other.

The one moment in which Simon Kinberg's script becomes almost subtle is when the couple have to go to their boring neighbours' party and one of the dull wives plonks a baby into the lap of the uncomfortable Angelina. An easy way to go with the scene would have been to show her face unwillingly lighting up with latent maternal joy, to underline how sympathetic her character essentially is, before having to suppress it. But actually she just looks horrified at the little bundle of joy in her arms. Kids are not her thing, not this movie's thing. End of story.

The sparks come when the couple are at open war, but still trying to sue for peace, and then back together again, with war threatening to re-erupt. When Angelina throws herself from the driver's seat of a moving car they're both in, in order to kill him, poor Brad is seen moaning, "We need to talk" out of the rear windshield as it crashes through a fence. Later, they have a painful discussion about their sexual track records. Brad's in the modest late 50s, Angelina's at 312, including two-at-a-time.

None of this is exactly the urbane, cosmopolitan comedy we could have expected from director Doug "Swingers" Liman, and the crash-bang action stuff - presumably there to interest the guys as well as their dates - could easily have come down by 15 minutes. It is moderate entertainment, though Angelina needs to find a script which can transform her eccentric style into something intentionally funny and Brad will have to work very hard to recover the poise he lost after that first Ocean's Eleven movie.